In a research paper co-authored by the present inventor and entitled "Quantitative Remote Sensing Tools for Air and Surface Measurements" presented and published at the Aug. 30, 1982 meeting of the International Society of Photogrammetry and Remote Sensing, there is included an outline description of a gas pipeline leak sensing instrument designed to detect methane and ethane leaking from natural gas pipelines. In this apparatus, incoming light is gathered by lenses and passed in two optical channels through a gas cell chopper wheel and band limiting interference filters onto cryogenically cooled infrared detectors. One optical train is used to detect methane, using a filter centred on a 7.8.mu. wavelength, and the other is used to detect ethane, using a filter centred on a 12.2.mu. wavelength. This radiometer makes use of a gas filter correlation technique, the chopper wheel containing a spectrally neutral filter, and cells containing samples of the gas to be detected.
The signals picked up by such a radiometer are considerably influenced by background radiation from the earth, atmospheric conditions, and fluctuations in the height above ground of an aircraft in which the instrument is flown caused by surface topography or other reasons. In theory, fluctuations in sensor output due to each of these causes can be compensated for, but in practice simultaneous compensation has proved difficult. Since methane is naturally present in the atmosphere at measurable concentrations, there are problems in distinguishing fluctuations in the methane signal due to leaks from fluctuations due to other causes, whilst typical ethane signals are often too small to detect with sufficient reliability.